The invention relates to a method for producing an aperture in a semiconductor material, for example (100)-oriented or polycrystalline silicon. Such apertures, whose size is in the micrometer range or below, are used, for example, as a component of probes for scanning near-field optical microscopes (SNOM). With this method, optical surface properties can be inspected with sub-wavelength resolution. As with any other scanning optical microscope, the resolution achievable by the scanning near-field optical microscope is limited by the geometry and the dimensions of the probe, which means in particular, the aperture and its distance from the surface of the sample piece. In order to achieve sub-wavelength resolution the light-emitting or detecting area of the probe must have lateral dimensions below 100 nm. In the prior art, there is no lack of attempts at producing such small dimensioned reproducible apertures in the 100 nm range or below. A method known from the prior art is shown schematically in FIG. 6. FIG. 6a reflects the cross-section through a semiconductor wafer 14 having an upper surface 16 and a lower surface 18. The upper surface 16 comprises a plurality of cavities 20, for example in the form of an inverse pyramid 30, preferably produced by means of anisotropic etching. Thereafter, the lower surface 18 of the semiconductor wafer 14, which consists of (100)-oriented silicon, for example, is etched back, particularly by means of anisotropic etching until the tips of the inverse pyramids are exposed, thus producing an aperture 10, as shown schematically by the first and second illustrations in FIG. 6b. The opening of the first aperture is too wide, the opening of the second aperture is ideal, while in the third example, the tip of the inverse pyramid has not yet opened at all.
This is due to the fact that the thickness of the semiconductor wafer varies highly. Even a variation in the thickness of only a few 10 nm can result in variations in the diameter or the cross-section of the aperture as shown in FIG. 6. This example, which is based on prior art, demonstrates that as a result of the variation in the thickness of the semiconductor wafer 14 only very few tips of the inverse pyramids have a suitable aperture size. Also, the size of said aperture is subject to high scattering.
Furthermore, studies known from the prior art on the oxidation behavior of silicon (Markus et al., Journal of the Electrochemical Society, Solid State Science and Technology, pages 1278-1282, 1982, and Kao et al., IEE Transactions on Electronic Devices, Volume 34, No. 5, page 1008, 1987 and Volume 35, No. 1, page 25, 1988) revealed a high dependence on the orientation in the plane of the semiconductor wafer, on the temperature and the structure of the surface. It was found that at low oxidation temperatures of approx. 800xc2x0 C. to 900xc2x0 C. the thickness of the oxide layer on convex and concave edges of the structured surface, for example in trench cells, decreases relative to the thickness of the oxide layer on the surface. These findings have already been used for producing very sharp silicon tips (Marcus et al, Applied Physics letters, 54 (3), pages 236-238, 1990) where curvature radii in the range of approx. 1 nm were achieved. A similar method for producing very sharp silicon tips for the so-called cantilever probes used in scanning probe microscopy is specified in EP-A-0468071.
Based on a method for producing an aperture in a semiconductor material comprising the steps of preparing a semiconductor wafer, for example a (100)-oriented semiconductor wafer having an upper surface and a lower surface, and producing a cavity with a side wall in the upper surface of the semiconductor wafer by partially etching said upper surface, the aim of the invention is to provide a method for producing an aperture whose size is below approx. 1 micrometer, particularly at approx. 100 nm, where the size of the aperture is adjustable so as to be reproducible.
The problem is solved with the method having the above mentioned characteristic features substantially in that the cavity comprises a closed bottom area, which faces the lower surface and which preferably has, in particular, a convex or, in particular, a concave corner or edge or a curvature of this type, that an oxide layer is deposited on the semiconductor material, at least in the area of the cavity by means of oxidizing the semiconductor material, where the oxide layer comprises an inhomogeneity, at least in the bottom area, that the semiconductor material is selectively etched back on the lower surface of the semiconductor wafer until at least the oxide layer located in the bottom area is exposed, and that the exposed oxide layer is etched until it is at least severed.
The method offers a particular advantage in that the measurement of the size of the aperture is not dependent on the variations in the layer thickness of the semiconductor wafer. The result is that the apertures to be produced are highly reproducible and thus they are able to open up new areas of application and resolutions, for example when they are used in probes in scanning near-field optical microscopy.
A number of procedural steps are performed for producing apertures in semiconductor materials, for example in (100)-oriented mono-crystalline silicon or polycrystalline silicon.
First, particularly pyramidal or other cavities are produced which are tapering at the lower end and which are etched into the semiconductor material. For this purpose, masking layers are provided on the surface of the semiconductor wafer. By means of optical or electron beam lithography and subsequent chemical, electrochemical or plasma etching methods the required structures are applied to the masking layer. The cavities are etched by means of wet-chemical or plasma etching methods. Alternatively, the cavities can also be produced by means of a focused ion beam. In the next step, the semiconductor material is oxidized, where the resulting oxide layer varies depending on the crystal orientation, the oxidation temperature and the curvature of the respective local structure of the surface of the semiconductor wafer. When suitable oxidation temperatures are selected the oxide layer has a higher etching rate in the places with the highest curvature as a result of stress effects, which means that the oxide layer, for example in the case of a tapering cavity, has one or more xe2x80x9cweak pointsxe2x80x9d in the area of the tip as relates to the etching process. In the following step, the oxide layer, which may have developed during the oxidation process on the lower surface of the semiconductor wafer, is removed using methods known in the art. Thereafter, the semiconductor material on the lower surface is etched back by wet-chemical etching or plasma etching until finally the tip of the oxide layer located in the cavity is exposed. It is important that a selective etching method is used for this so as to fully or at least largely prevent that the oxide layer is also etched. The semiconductor material is etched back until the single or all of the oxide layers, for example of an array of cavities, are exposed. As a result of variations in the thickness of the semiconductor wafer it is quite possible that the multiple tips, if applicable, of the oxide layer will project to a greater or lesser degree from the lower surface of the semiconductor wafer.
This is not problematic for dimensioning the aperture size, however, insofar as the tips of the oxide layer which project to a greater or lesser degree all have substantially the same form, at least with regard to the thickness and form of the oxide layer, and particularly in the area of the tip, they each have one or more weak points. Thereafter, the oxide layer is thinned with an etching agent, which is selective with regard to the material of the oxide layer, until the oxide layer breaks through on the xe2x80x9cweak pointsxe2x80x9d of the oxide layer and the desired apertures are produced in the oxide stumps. The etching process is then discontinued unless larger apertures are desired.
Accordingly, this method produces miniaturized apertures of a well defined size over the entire semiconductor wafer. If the etching process is continued, however, the stump of the oxide layer will also be etched, thereby achieving apertures whose size can be adjusted over the length of the etching process.
There is also an option of providing the cavities with an edge or a plateau in the bottom area so as to produce two or four openings per cavity in the oxide layer in the area of the tips using the above described method.
The reproducibility of the method is based on the knowledge of utilizing the special oxidation properties, for example of (100)-oriented silicon wafers for producing reproducible apertures of identical size over the full substrate surface despite variations in the thickness of the substrate. For this purpose, the substrate comprising the cavity is oxidized at approx. 800xc2x0 C. to 900xc2x0 C. so as to produce an oxide layer thickness with an inhomogeneous etching rate and layer thickness.
The oxide is thinnest in the places with the highest curvature, which means that the thinned oxide layer sections are disposed in the area of the tips of the cavities. The oxide layers in the multiple cavities are exposed by selectively etching back the semiconductor material on the lower surface of the semiconductor wafer. The oxide layer is not or only marginally affected by this step. In this phase of the production process, all of the cavities shaped by the oxide layer have an identical oxide layer structure. In particular, the oxide layer structure is independent of variations in the thickness of the semiconductor material. All substantially identical exposed tips of the oxide layer can be thinned or removed in a subsequent step using a selective etching agent, until the oxide layer of all tips breaks through in the same place, i.e. the weak point of the oxide layer, and the tip of the oxide layer breaks off. With this method, apertures having virtually the same size are obtained in all oxide stumps over the entire surface of the semiconductor wafer.
According to an advantageous embodiment of the invention, the cavities have the shape of an inverse pyramid or a V-shaped channel or an inverse pyramid stump or the shape of a plateau.
In particular, a plurality of cavities is disposed, for example in the form of an array, on the surface of the semiconductor wafer.
It is particularly advantageous for the oxide layer to comprise an inhomogeneity in the bottom area in the form of one or more weak points or taperings.
Advantageously, the surface of the oxide layer is subjected to a metallization process, particularly with aluminum, after it has been severed. This offers the option of specifically reducing the aperture which is already present in the oxide layer. Metallization also improves the optical properties of a sensor which is provided with such an aperture.
Advantageously, the diameter of the aperture is in the range of approx. 100 nm or below.
According to another advantageous embodiment of the method, the cavities are preferably produced by means of anisotropic etching.
The oxide layer is produced by means of heating the semiconductor wafer to approx. 900xc2x0 C., particularly in a humid atmosphere and preferably for approx. 2 hrs.
The semiconductor material is selectively etched back isotropic or anisotropic, for example by means of a KOH solution, in particular approx. 40 percent by weight, preferably at approx. 60xc2x0 C.
The semiconductor material is preferably selectively etched back until the oxide layers of preferably substantially all or at least most of the cavities of the semiconductor wafer are exposed.
The exposed oxide layer is preferably etched with ammonium fluoride buffer, in particular 1 buffer:16 water.
The size of the aperture is varied so as to be increased substantially by the etching time for the oxide layer after it has been severed.
According to another advantageous embodiment the cavities are configured edge-shaped or plateau-shaped, where two or four apertures are produced per each cavity.
The invention also relates to an aperture in a semiconductor material, which is produced in particular according to any of the procedural claims, where the aperture is formed by an oxide layer located on an inside wall of a break-through in the semiconductor material.
According to an embodiment of the invention, a semiconductor layer and/or an organic material layer and/or a metal layer, particularly an aluminum layer, is deposited on the oxide layer.
It was found to be advantageous if the oxide layer is formed of an oxide of the semiconductor material.
The invention also relates to the use of an aperture which is characterized in that the semiconductor material and the aperture are integrated particularly in the front area of a bending arm which is clamped in on one side, particularly a so-called cantilever.
An advantageous embodiment of said use is that a single bending arm or a plurality of bending arms is used as a sensor element in a matrix arrangement.
According to another advantageous embodiment, the bending arm or bending arms are used as sensor elements in scanning probe microscopy.
By depositing a thin, optically low transparent layer, it was found to be advantageous to use the bending arm or bending arms for simultaneous scanning force microscopy (AFM, SFM) or for scanning near-field optical microscopy (SNOM) where, when the aperture is illuminated from the surface of the semiconductor wafer, the aperture is used as a miniaturized light source in the optical near-field range (the so-called illumination mode), or where by the aperture itself luminous power is collected from the illuminated sample piece (the so-called collection mode).
By sequentially depositing materials, such as metal, semiconductors, polymers on the front and/or the rear of the bending arm a miniaturized contact place of the materials is obtained at the place of the aperture.
A further advantageous use is a matrix-like arrangement of apertures, for example in the form of an array on flat substrates or structured surfaces (for example cantilevers), which is used for sorting particles according to size in the manner of a sieve.
Another use is characterized in that a particularly matrix-like arrangement of one or more apertures on flat substrates or on structured surfaces (for example cantilevers) is used for apportioning and/or injecting precise very small amounts of liquid or gas.
Additional characteristic features, advantages, applications and objectives of the invention are found in the following description of exemplary embodiments based on the drawings. All characteristic features which are described and/or graphically depicted separately or in any combination represent the subject matter of the invention, irrespective of any summarization in the claims or references thereto.